1. Field of the Invention
The present invention relates to an antenna device and a communication terminal device that are preferably used for a communication system using a high-frequency (HF) band or an ultra-high-frequency (UHF) band.
2. Description of the Related Art
In a device that is included in an electronic apparatus, such as a cellular phone, and performs HF-band communication, such as a near field communication (NFC), a radio-frequency integrated circuit (RFIC) and a matching element are typically mounted on a circuit substrate, an antenna is bonded to the inner side of a housing of the electronic apparatus, and the RFIC and the antenna are electrically connected to each other with a spring pin or the like interposed therebetween.
Meanwhile, a reduction in the thickness of a recent wireless communication terminal such as the cellular phone terminal has advanced, and “metallization” by applying magnesium plating to a housing and so on is performed in increasing cases in order to compensate for insufficient strength associated with such reduction in thickness.
However, when the “metallization” is performed on the terminal housing, an electromagnetic field around an antenna embedded in the terminal is shielded by metal. Due to the shielding of the electromagnetic field, there arises a problem that the antenna cannot make communication with a party-side antenna.
In order to solve the above problem, as disclosed in Japanese Patent No. 4993045, an antenna device having a configuration in which a metal plate having a larger area than an antenna coil proximate (magnetic field coupling) to the antenna coil and the metal plate is used as a radiator has been proposed.
By employing the antenna configuration disclosed in Japanese Patent No. 4993045, the antenna can make communication with a party-side antenna although the antenna is covered with metal. However, with provision of a slit or an opening in the metal plate, a decrease in the mechanical strength needs to be considered, and the number of processes in manufacturing increases. Further, when a slit or an opening is to be provided in a metal housing, in particular, design of the housing is restricted. In addition, the vicinity of the slit or the opening cannot be connected to the ground of a circuit. Therefore, the potential of the metal plate partially fluctuates in some cases, resulting in a problem that an electric field shielding effect of the metal plate decreases or a concern over interference of a first conductor surface and a second conductor surface with another high-frequency circuit is generated.